Composer is a 3rd party tool for dependency management in PHP but can be used to manage Drupal 8 modules and etc.

To install, open up a terminal and SSH to your server:

Change to /tmp directory

cd /tmp

Download the composer.phar file using this command:

curl -sS https://getcomposer.org/installer | php

Move composer.phar to /usr/local/bin/, using this command:

mv composer.phar /usr/local/bin/composer

Now you should be able to use the command composer globally.

To test your installation, run:

composer

Composer uses packagist.org to find open source libraries to manage and install.

You can also install Drush using Composer easily. Be sure to install Drush 8.x because this is the version with Drupal 8 support (i use it for Drupal 7 as well so no need to install multiple version of Drush).